Lithium secondary battery

ABSTRACT

There is provided a lithium secondary battery with a negative electrode which comprises a negative electrode active material layer comprising alloy particles comprising silicon and tin and having an average particle diameter of 0.05 to 2 μm as an active material, and a negative electrode current collector, wherein the negative electrode active material layer has a storage capacity of 1,000 to 2,200 mAh/g and a density of 0.9 to 1.5 g/cm 3  and which thereby has a high capacity and a good cycle-characteristic. Thus, a lithium secondary battery having a high capacity and a long life and so designed as to exhibit these characteristics at the same time is provided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a lithium secondary battery,more particularly improvement of the capacity and cycle-characteristicof a lithium secondary battery.

[0003] 2. Related Background Art

[0004] The so-called lithium secondary batteries comprising a positiveelectrode with lithium cobaltate as a major active material, negativeelectrode with carbon as a major active material and an organicelectrolyte solution have been put on the markets since the beginning ofthe 1990's. They have been rapidly spreading in the markets since then,because of their higher capacity than that of a conventionalnickel/hydrogen secondary battery and sufficient cycle-characteristic tosatisfy the market needs. At the same time, extensive works have beendone to improve their characteristics and develop batteries of highercapacities.

[0005] As a result, the cylindrical battery of 18 mm in diameter and 65mm in height, the so-called 18650 size, has now a capacity of 2,200 mAhat the highest, comparing with around 1,000 mAh recorded in thebeginning of the 1990's. The greatly enhanced capacity results fromimprovements in a wide area including materials, e.g., lithium cobaltateand carbon as active.materials, and designs.

[0006] However, it is considered that current capacity of a lithium ionsecondary battery with lithium cobaltate and carbon as major activematerials is close to the limit. Therefore, new active materials havebeen studied for the positive and negative electrode as another approachto higher capacity.

[0007] In particular, for the negative electrode active materials,metallic materials that can be alloyed with lithium, e.g., silicon andtin, have been studied as substitutes for carbon materials such asgraphite. This is because they have greater theoretical capacities whichare 3 to 10 times that of graphite such that while the theoreticalcapacity capable of charging/discharging of graphite is 372 mAh/g, asilicon alloy (Li_(4.4)Si) has a theoretical capacity of 4,199 mAh/g anda tin alloy (Li_(4.4)Sn) has a theoretical capacity of 993 mAh/g.

[0008] However, some metallic materials that can be alloyed with lithiuminvolve their own problems to be solved, because they may expand duringthe alloying reaction process to increase the negative electrode volumeseveral times, which tends to powder them, resulting in deterioration oftheir cycle-characteristic.

[0009] Several proposals have been made to solve these problems, asdisclosed by U.S. Pat. Nos. 6,051,340, 5,795,679 and 6,432,585, JapanesePatent Application Laid-Open Nos. 11-283627 and 2000-311681 and WO00/17949.

[0010] For example, U.S. Pat. No. 6,051,340 proposes a lithium secondarybattery with a negative electrode comprising a current collector coatedwith an electrode layer, wherein the current collector is of a metalwhich is not alloyed with lithium, and the electrode layer comprises ametal which can be alloyed with lithium, such as silicon or tin andanother metal which is not alloyed with lithium, such as nickel orcopper.

[0011] U.S. Pat. No. 5,795,679 proposes a lithium secondary battery witha negative electrode formed of an alloy powder comprising an elementsuch as nickel or copper and another element such as tin; and U.S. Pat.No. 6,432,585 a battery with a negative electrode whose electrodematerial layer contains at least 35% by weight of silicon or tinparticles having an average particle diameter of 0.5 to 60 μm, a voidratio of 0.10 to 0.86 and a density of 1.00 to 6.56 g/cm³.

[0012] Japanese Patent Application Laid-Open No. H11-283627 proposes alithium secondary battery with a negative electrode containing siliconor tin having an amorphous phase; and Japanese Patent ApplicationLaid-Open No. 2000-311681 a lithium secondary battery with a negativeelectrode composed of amorphous tin/transition metal alloy particles ofa non-stoichiometric composition. WO 00/17949 discloses a lithiumsecondary battery with a negative electrode composed of amorphoussilicon/transition metal alloy particles of a non-stoichiometriccomposition.

[0013] Moreover, Japanese Patent Application Laid-Open No. 2000-215887proposes a lithium secondary battery whose capacity and charge/dischargeefficiency are improved by suppressing the volume expansion duringalloying with lithium to prevent the breakage of the negative electrode,wherein chemical vapor deposition involving pyrolysis of benzene or thelike is used to solve the above problems by forming a carbon layer onthe surface of particles of a metal or semi-metal, in particularsilicon, which can be alloyed with lithium, to improve itselectroconductivity.

[0014] These inventions have disclosed compositions and constituents ofsilicon or its alloys, and performance of the electrode that comprisesthe above material. It should be noted, however, that a battery exhibitsits inherent functions when its negative electrode works in combinationwith a positive electrode, both contained in a battery can. For abattery to exhibit its intended functions, it is essential to design abattery of high capacity and cycle-characteristic by allowing a negativeelectrode mainly composed of a metallic material which can be alloyedwith lithium to effectively function in a battery can in combinationwith a positive electrode.

[0015] Japanese Patent Application Laid-Open No. 2002-352797 proposes alithium secondary battery of high capacity and cycle-characteristic bycontrolling utilization of a negative electrode comprised of silicon ata certain level or less. However, it only discloses silicon coated withcarbon for a negative electrode, discussing that recommended electricalstorage capacity (hereinafter, simply referred to as “storage capacity”or “capacity”) per unit weight of a negative electrode active materiallayer is 1,000 mAh/g, but is silent on conditions for extending the lifeof a battery having a capacity exceeding 1,000 mAh/g.

[0016] In other words, few have sufficiently discussed optimum electrodeand battery designs that allow a battery to exhibit a high capacity anda long cycle life when it comprises a negative electrode of a highcapacity per unit weight of a negative electrode active material layerexceeding 1,000 mAh/g working in combination with a positive electrode.

SUMMARY OF THE INVENTION

[0017] The present invention has been accomplished in the light of theabove-mentioned situation, and it is an object of the present inventionto provide a lithium secondary battery having a high capacity and a longlife and so designed as to exhibit these characteristics at the sametime.

[0018] The present invention provides a lithium secondary battery with anegative electrode comprising a negative electrode active material layercomprising alloy particles comprising silicon and tin and having anaverage particle diameter of 0.05 to 2 μm as an active material, and acurrent collector, wherein the negative electrode active material layerhas a storage capacity of 1,000 to 2,200 mAh/g and a density of 0.9 to1.5 g/cm³.

[0019] The present invention also provides a lithium secondary batterycomprising a negative electrode comprising a negative electrode activematerial layer comprising alloy particles as an active materialcomprising silicon as a major component and a negative electrode currentcollector, and a positive electrode comprising a positive electrodeactive material layer and a positive electrode current collector,wherein the positive electrode active material layer and the negativeelectrode active material layer satisfy the following relationships:

(C _(N) ×D _(N))/(C _(P) ×D _(P))≦8

C _(N) ×D _(N)=1,200 to 2,500 mAh/cm³

C_(N)=1,000 to 2,200 mAh/g

D_(N)=0.9 to 1.5 g/cm³

[0020] wherein,

[0021] C_(N) represents a capacity per unit weight of the negativeelectrode active material layer;

[0022] D_(N) represents the density of the negative electrode activematerial layer;

[0023] C_(P) represents a capacity per unit weight of the positiveelectrode active material layer; and

[0024] D_(P) represents the density of the positive electrode activematerial layer.

[0025] In the present invention, it is preferred that the alloyparticles comprising silicon as a major component have an averageparticle diameter of 0.05 to 2 μm.

[0026] In the present invention, it is also preferred that the alloyparticles comprising silicon as a major component are alloy particlescomprising silicon and tin.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a sectional view schematically showing an embodiment ofthe secondary battery (lithium secondary battery) of the presentinvention;

[0028]FIG. 2 is a graphical representation showing the relationshipbetween the capacity per unit weight (inserted Li amount) and thechanges in cycle life and expansion coefficient of the negativeelectrode active material layer of the above-mentioned secondary batterywherein a silicon/tin alloy powder is used as an active material;

[0029]FIG. 3 is a graphical representation showing the relationshipbetween the density and the changes in cycle life and expansioncoefficient of the negative electrode active material layer of theabove-mentioned secondary battery wherein a silicon/tin alloy powder isused as an active material;

[0030]FIG. 4 is a view showing the design range for the positiveelectrode active material layer and the negative electrode activematerial layer of the above-mentioned secondary battery; and

[0031]FIG. 5 is a sectional view showing the structure of a spiral-woundtype cylindrical battery as one embodiment of the above-mentionedsecondary battery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The preferred embodiments of the present invention will bedescribed by referring to the attached drawings.

[0033] The present inventors have developed a negative electrode with anunprecedentedly high capacity by using alloy particles as an activematerial comprising silicon as a major component, and found thoseelectrode conditions under which the electrode performance (includingcycle-characteristic) and the capacity are well-balanced. Further, theyhave found the optimum electrode and battery design conditions whichallow a battery with a negative electrode of high capacity working incombination with a positive electrode to exhibit high capacity and longlife. The secondary batteries of preferred embodiments of the presentinvention will be described in detail.

[0034]FIG. 1 is a sectional view schematically showing an embodiment ofthe secondary battery (lithium secondary battery) of the presentinvention. In the lithium secondary battery, a negative electrode 103comprising a negative electrode active material layer 102 comprisingalloy particles as an active material comprising silicon as a majorcomponent and formed on a negative electrode current collector 101, anda positive electrode 106 comprising a positive electrode active materiallayer 105 comprising lithium-containing transition metal oxide as anactive material and formed on a positive electrode current collector 104are stacked in opposition to each other via an ion conductor 107 andcontained in a battery case 112, and the negative electrode 103 isconnected to a negative electrode terminal 108 via a negative electrodelead 110 and the positive electrode 106 is connected to a positiveelectrode terminal 109 via a positive electrode lead 111.

[0035] The present inventors have developed a negative electrode with ahigh capacity and a good cycle-characteristic by using alloy particlesas an active material comprising silicon as a major component, and foundthe optimum capacity and density for battery design.

[0036]FIG. 2 is a view illustrating the relationship between thecapacity per unit weight (inserted Li amount) and the changes in cyclelife and expansion coefficient of a negative electrode active materiallayer 102 using a silicon/tin alloy powder an active material having anaverage particle diameter of 0.05 to 2 μm is used as an example of thealloy.

[0037] The battery was evaluated by the charging/discharging test,wherein the negative electrode 103 was used as a cathode and metalliclithium was used as an anode in an electrolyte solution of 1 M (mol/L)prepared by dissolving lithium hexafluorophosphate (LiPF₆) in a mixtureof ethylene carbonate (EC) and diethyl carbonate (DEC) in equivalentamounts.

[0038] The charging/discharging test was carried out with a cycleconsisting of a Li insertion/release reaction at a current density of 1mA/cm² and a 20-minutes resting period being defined as one cycle. TheLi insertion reaction was cut off at a given capacity or 0 V, and the Lirelease reaction was carried out with the cut-off voltage being set to1.2 V. Incidentally, the electrode exhibited a storage capacity of 2,400mAh/g at the maximum by continuing the Li insertion reaction to 0 V.

[0039] The expansion coefficient was measured after the Li insertionreaction in a first cycle, and the cycle life was evaluated on the basisof the number of cycles in which the capacity did not reach 60% of thegiven capacity. Incidentally, the life shown in FIG. 2 was valuesnormalized with the number of cycles at a given capacity of 1,000 mAh/gbeing defined as 1.0.

[0040]FIG. 3 is a view illustrating the relationship between the densityof a negative electrode active material layer 102 and the changes incycle life and expansion coefficient when effecting charging anddischarging at a capacity per unit weight of the negative electrodeactive material layer 102 1,400 mAh/g. The evaluation was carried out inthe same manner as that described for FIG. 2. Further, the life shown inFIG. 3 was values normalized with the number of cycles at a density of1.0 g/cm³ being defined as 1.0.

[0041] It can be seen from the results shown in FIGS. 2 and 3 that theexpansion coefficient of the negative electrode active material layer102 using a silicon/tin alloy powder as an active material becomesgreater as the capacity increases (i.e., as the quantity of lithiuminserted increases) and as the density of the negative electrode activematerial layer 102 increases. Thus, there is a tendency that thenegative electrode active material layer 102 is liable to generatestrain or cracks to reduce the current collectability, therebydeteriorating the cycle-characteristic. Conversely, there is a tendencythat the expansion coefficient becomes smaller as the capacity decreases(i.e., as the quantity of lithium inserted decreases) and as the densityof the negative electrode active material layer 102 decreases, therebyimproving the cycle-characteristic.

[0042] These results lead to the conclusion that the preferable capacityper unit weight of the negative electrode active material layer 102 iswithin the range of 1,000 to 2,200 mAh/g. The reason is that exceeding2,200 mAh/g significantly deteriorates the cycle-characteristic due toexpansion, which is not desirable. Further, although there are caseswhere priority is given to improvement in the cycle-characteristic atsome degree of sacrifice of the capacity, no improvement in thecycle-characteristic is expected at a capacity below 1,000 mAh/g.Incidentally, the preferable capacity per unit weight of the activematerial is within the range of 1,500 to 3,000 mAh/g, although varyingdepending on the composition of the active material layer.

[0043] On the other hand, the preferable density of the negativeelectrode active material layer 102 is within the range of 0.9 to 1.5g/cm³. The reason is that exceeding 1.5 g/cm³ significantly deterioratesthe cycle-characteristic due to expansion, which is not desirable.Further, when the density is degreased, the battery capacity inevitablydecreases. Incidentally, although there are cases where priority isgiven to improvement in the cycle-characteristic at some degree ofsacrifice of the battery capacity, no improvement in thecycle-characteristic has been attained at a density below 0.9 g/cm³.

[0044] Thus, the present inventors have found from the results shown inFIGS. 2 and 3 that the capacity per unit volume of the negativeelectrode active material layer 102 expressed by a product of capacityper unit weight and density is preferably within the range of 900 to3,300 mAh/cm³, more preferably 1,200 to 2,500 mAh/cm³. The reason isthat exceeding 2,500 mAh/cm³ significantly deteriorates thecycle-characteristic due to expansion, and that although there are caseswhere priority is given to improvement in the cycle-characteristic atsome degree of sacrifice of the capacity, no improvement in thecycle-characteristic has not been attained at a capacity below 1,200mAh/cm³.

[0045] In addition, based on the results shown in FIGS. 2 and 3, thepresent inventors have found as a second specific feature of the presentinvention that the negative electrode comprising alloy particles as anactive material comprising silicon as a major component to provide theabove-mentioned high capacity can work in combination with a positiveelectrode to fully exhibit its inherent functions, by designing thepositive electrode active material layer 105 and the negative electrodeactive material layer 102 so as to satisfy the following relationships:

(C _(N) ×D _(N))/(C _(P) ×D _(P))≦8

C _(N) ×D _(N)=1,200 to 2,500 mAh/cm³

C_(N)=1,000 to 2,200 mAh/g

D_(N)=0.9 to 1.5 g/cm³

[0046] wherein,

[0047] C_(N) represents a capacity per unit weight of the negativeelectrode active material layer;

[0048] D_(N) represents the density of the negative electrode activematerial layer;

[0049] C_(P) represents a capacity per unit weight of the positiveelectrode active material layer; and

[0050] D_(P) represents the density of the positive electrode activematerial layer.

[0051] A combination of the positive electrode active material layer 105and the negative electrode active material layer 102 that satisfy theabove relationships can provide a battery of a high capacity and anexcellent cycle-characteristic.

[0052]FIG. 4 is a view illustrating these relationships in detail, wherethe area enclosed by the boundaries 1 to 3 is a range in which a batteryof a high capacity and an excellent cycle-characteristic can beprovided. Incidentally, in FIG. 4, the abscissa indicates a capacity perunit volume of the positive electrode active material layer that can berepeatedly charged and discharged, represented by C_(P)×D_(P), and theordinate indicates (C_(N)×D_(N))/(C_(P)×D_(P)).

[0053] Here, the term“capacity per unit volume of the positive electrodeactive material layer 105 that can be repeatedly charged and discharged”means a reaction region in which the charge/discharge cycles are wellcarried out and obtained as a product of the capacity per unit weightand the density of the positive electrode active material layer 105.Further, the capacity per unit weight of the positive electrode activematerial layer 105 can be determined by the capacity per unit weight ofthe positive electrode active material that can be repeatedly chargedand discharged and the compositional ratio by weight of the positiveelectrode active material contained in the positive electrode activematerial layer 105.

[0054] For example, the positive electrode active material layer 105that can be repeatedly charged and discharged of the cobalt-, nickel-and manganese-type lithium-containing transition metal oxides, which canprovide a high voltage, is practically used at a capacity per unitvolume of 200 to 700 mAh/cm³, a capacity per unit weight of 80 to 200mAh/g and a density of 2.5 to 3.5 g/cm³.

[0055] More specifically, as the active material layer using theselithium-containing transition metal oxides used for commerciallyavailable batteries, an LiCoO₂ active material layer is practically usedat a capacity per unit weight of 140 to 160 mAh/g and a density of 3.0to 3.5 g/cm³; an LiNiO₂ active material layer, which has a highertheoretical capacity than that of LiCoO₂, is practically used at acapacity per unit weight of 170 to 200 mAh/g and a density of 2.8 to 3.2g/cm³, and an LiMn₂O₄ active material layer is practically used at acapacity per unit weight of 80 to 120 mAh/g and a density of 2.5 to 3.0g/cm³ for.

[0056] The boundary 1 represents the values of(C_(N)×D_(N))/(C_(P)×D_(P)) vs. C_(P)×D_(P) that is the capacity perunit volume of the positive electrode active material layer which can berepeatedly charged and discharged when the C_(N)×D_(N) is the minimumvalue 1,200 mAh/cm³. Although there are cases where priority is given tothe cycle-characteristic at the sacrifice of capacity to some extent,any value below the boundary 1 is not desirable, because no improvementin the cycle-characteristic is expected and the high capacityperformance of the negative electrode cannot be fully exhibited.

[0057] The boundary 2 represents the values of(C_(N)×D_(N))/(C_(P)×D_(P)) vs. C_(P)×D_(P) that is the capacity perunit volume of the positive electrode active material layer which can berepeatedly charged and discharged when the C_(N)×D_(N) is the maximumvalue 2,500 mAh/cm³ Any value exceeding the boundary 2 is not desirable,because the cycle-characteristic lowers.

[0058] The boundary 3 represents (C_(N)×D_(N))/(C_(P)×D_(P))=8. Valueson or below the boundary 3 can provide a more stable battery and henceare preferable. The reason is as described below. Generally, thepositive electrode active material layer 105 and the negative electrodeactive material layer 102 facing each other satisfy the followingrelationship.

C_(N)×D_(N)×T_(N)=C_(P)×D_(P)×T_(P)  Equation 1

[0059] The following Equation 2 is derived from Formula 1.

(C _(N) ×D _(N))/(C _(P) ×D _(P))=T _(P) /T _(N)  Equation 2

[0060] In the above equations, T_(N) represents the thickness of thenegative electrode active material layer 102; C_(N) the capacity perunit weight of the negative electrode active material layer 102; D_(N)the density of the negative electrode active material layer 102; T_(P)the thickness of the positive electrode active material layer 105; C_(P)the capacity per unit weight of the positive electrode active materiallayer 105; and D_(P) the density of the positive electrode activematerial layer 105.

[0061] As is seen from Equation 2, the (C_(N)×D_(N))/(C_(P)×D_(P)) canbe represented in terms of the ratio of the active material layersT_(P)/T_(N). The practical thickness of the positive electrode activematerial layer 105 in consideration of the battery characteristics,adhesion to the current collector and productivity, is 150 μm or less,more preferably 100 μm or less. With this positive electrode activematerial layer 105, if the boundary 3 is exceeded, the negativeelectrode active material layer 102 is too thin, which makes itdifficult to effect uniform coating in mass production to lower theproductivity. Further, during electrode group formation steps such asstacking or rolling, rolling in a skewed fashion or the like is liableto occur. Therefore, the (C_(N)×D_(N))/(C_(P)×D_(P)) value is preferably8 or less.

[0062] As described above, as long as the conditions defined by thepresent invention and shown in FIG. 4 are satisfied, it is possible tostably provide a battery suitable for a specified object of use, such asa battery featuring a high capacity while securing the life to a certainextent, or a battery featuring a long life while still having a highercapacity than a commercially available battery with a graphite negativeelectrode.

[0063] Next, the negative electrode 103, negative electrode currentcollector 101, negative electrode terminal 108, positive electrode 106,positive electrode current collector 104, positive electrode terminal109, and ion conductor 107 of the secondary battery (lithium secondarybattery) mentioned above with reference to FIG. 1 will be described.(Negative Electrode 103) The negative electrode 103 is generallyconsisted of the negative electrode current collector 101, and thenegative electrode active material layers 102 formed on both sides ofthe negative electrode current collector 101. The negative electrodeactive material layer 102 is constituted of alloy particles mainlycomposed of silicon, a conductive auxiliary material, other additives,and a binder for holding the active material layers on each other or forholding the active material layer on the current collector.

[0064] For example, the negative electrode active material layer 102 isformed by suitably adding a conductive auxiliary material and a binderto the alloy particles mainly comprised of silicon, followed by mixing,application and pressure forming. Further, it is preferable to add asolvent to the above mixture to prepare a paste, for facilitating theapplication. As the application method, a coater coating method orscreen printing method may be used.

[0065] Alternatively, the negative electrode active material layers 102can be formed on the current collector by pressing and forming thenegative electrode main material, a conductive auxiliary material and abinder on the current collector without addition of a solvent, orpressing and forming the negative electrode main material and aconductive auxiliary material on the current collector without additionof a binder. Incidentally, the content of the alloy particles mainlycomposed of silicon in the negative electrode active material layer 102is preferably 40 to 90% by weight.

[0066] As the active material of the negative electrode 103, there arepreferably used an alloy powder mainly composed of silicon that arestable in an electrolyte solution and capable of insertion/release oflithium. It is important that the composition of the alloy powder mainlycomposed of silicon is such that the silicon content is preferably 50atomic % or more, more preferably 50% by weight or more. As componentelements other than silicon of the silicon alloy, there is preferablyused at least one element selected from the group consisting of tin,aluminium, zinc, germanium, indium, antimony, titanium, chromium, lead,copper, nickel, cobalt and iron.

[0067] These alloy powders are preferably amorphous. The averagediameter of the alloy particles is preferably 2 μm or less, morepreferably 0.9 μm or less, and more preferably 0.05 μm or more.

[0068] The binder used in the present invention is not particularlylimited, so long as it is electrochemically and chemically stable and isadhesive. Examples of the binder include water-insoluble polymers suchas polyolefins such as polyethylene and polypropylene, fluororesins suchas polyvinylidene fluoride, tetrafluoroethylene polymer and vinylidenefluoride-hexafluoropropylene copolymer, polyethylene-polyvinyl alcoholcopolymer, and styrene-butadiene rubber; and water-soluble polymers suchas polyvinyl alcohol, polyvinyl butyral, polyvinyl methyl ether,polyvinyl ethyl ether, polyvinyl isobutyl ether, carboxymethylcellulose,hydroxyethylcellulose, hydroxypropylcellulose,hydroxymethylethylcellulose, polyethylene glycol, and styrene-butadienerubber.

[0069] A preferable water-soluble polymer is a mixture of polyvinylalcohol and a cellulosic polymer, preferably carboxymethylcellulose.

[0070] Here, the content of the binder in the active material layer ispreferably 1 to 20% by weight in order to hold a larger quantity of theactive material at the time of charging, and more preferably 5 to 15% byweight. This is because the alloy powder more expands than carbon powderduring charging, so that a greater binding force is needed.

[0071] As the conductive auxiliary material, there is preferably usedthose materials which are electrochemically and chemically stable andwhose conductivity is as large as possible. Preferable examples of theconductive auxiliary material include carbon powder (in particulargraphite powder), copper powder, nickel powder, aluminium powder andtitanium powder.

[0072] (Negative Electrode Current Collector 101)

[0073] The material of the negative electrode current collector 101needs to be electrochemically and chemically stable, highly conductive,and not alloyed with lithium and includes, for example, copper, nickel,stainless steel and titanium. The current collector may be sheet-shaped,net-shaped, expanded, perforated or spongy. The current collector ispreferably 6 to 30 μm in thickness. When the thickness is less than 6μm, although the battery capacity will increase, the resistance of thecurrent collector will increase to result in an increase of internalresistance, a lowering in output, which is not desirable. On the otherhand, when the thickness is more than 30 μm, the battery capacity willdecrease, which is also not desirable.

[0074] (Negative Electrode Terminal 108)

[0075] The material of the negative electrode terminal 108 includescopper, nickel and stainless steel, as is the case with the negativeelectrode current collector 101. The method of connecting the negativeelectrode current collector 101 and the negative electrode terminal 108to each other includes laser welding, resistance welding, ultrasonicwelding or the like, and is suitably selected depending on the materialused.

[0076] (Positive Electrode 106)

[0077] The positive electrode 106 is generally constituted of thepositive electrode current collector 104, and the positive electrodeactive material layers 105 formed on both sides of the positiveelectrode current collector 104. The positive electrode active materiallayer 105 is constituted of an active material powder capable ofinsertion/release of lithium, a conductive auxiliary material, otheradditives, and a binder for holding the active material powder on eachother or for holding the active material powder on the currentcollector.

[0078] Here, the thickness of the positive electrode active materiallayer 105 to be formed on one side of the positive electrode currentcollector 104 is preferably 50 to 150 μm. Incidentally, when thethickness is less than 50 μm, because the capacity per unit volume ofthe negative electrode active material layer 102 is greater than that ofthe positive electrode active material layer 105, the thickness of theopposing negative electrode active material layer 102 becomes too smallto cause difficulty in coating, which is not desirable. On the otherhand, when the thickness is more than 150 μm, a lowering in the adhesionwith the positive electrode current collector 104 or a lowering in theoutput characteristic resulting from increase in polarization arises,which is not desirable.

[0079] On the other hand, the density of the positive electrode activematerial layer 105 is preferably 2.5 to 3.5 g/cm³. When the density isless than 2.5 g/cm³, the capacity increase of the battery cannot beachieved, which is not desirable. On the contrary, when the density ismore than 3.5 g/cm³, the electrolyte solution cannot effect sufficientpenetration or the active material layer is cracked to drop off from thecurrent collector when rolling the electrodes, which is also notdesirable.

[0080] Incidentally, the active material of the positive electrode 106is not particularly limited so long as it is stable in the electrolytesolution and capable of insertion/release of lithium and includes, forexample, transition metal oxides, transition metal sulfides, transitionmetal nitrides, lithium-containing transition metal oxides,lithium-containing transition metal sulfides, lithium-containingtransition metal nitrides, and lithium-containing transition metalphosphates, as preferable ones. Of these, lithium-containing transitionmetal oxides are more preferred. The transition metal element for thetransition metal oxides, transition metal sulfides, transition metalnitrides or transition metal phosphates includes, for example, metalelements having a d-shell or f-shell, i.e., Sc, Y, lanthanoids,antinodes, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co,Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au. In particular, Mn, Fe, Co and Ni ofthe first transition series metals are preferably used. Morespecifically, LiCoO₂, LiNiO₂, LiMn₂O₄, LiMnO₂, LiCo_(0.2)Bi_(0.8)O₂,LiNi_(0.5)Mn_(5.0)O₂ and the like may be used.

[0081] The binder is not particularly limited, so long as it iselectrochemically and chemically stable and is adhesive. Widely usedexamples of the binder include fluororesins such aspolytetrafluoroethylene and polyvinylidene fluoride, and cellulose-typeones such as carboxymethylcellulose and vinyl acetate-type ones such aspolyvinyl alcohol are also used.

[0082] As the conductive auxiliary material, there is preferably usedthose materials which are electrochemically and chemically stable andwhose conductivity is as large as possible. Preferable examples of theconductive auxiliary material include carbon powder (in particulargraphite powder), aluminium powder and titanium powder. (PositiveElectrode Current Collector 104) The material of the positive electrodecurrent collector 104 needs to be electrochemically and chemicallystable and highly conductive, and includes, for example, aluminium andtitanium. The current collector may be sheet-shaped, net-shaped,expanded, perforated or spongy. The positive electrode current collector104 is preferably 6 to 30 μm in thickness. Incidentally, when thethickness is less than 6 μm, although the battery capacity willincrease, the resistance of the positive electrode current collector 104will increase to result in an increase of internal resistance, alowering in output, which is not desirable. On the other hand, when thethickness is more than 30 μm, the battery capacity will decrease, whichis also not desirable.

[0083] (Positive Electrode Terminal 109)

[0084] The material of the positive electrode terminal 109 includesaluminium and titanium, as is the case with the positive electrodecurrent collector 104 of the positive electrode 106. The method ofconnecting the positive electrode current collector 104 of the positiveelectrode 106 and the positive electrode terminal 109 to each otherincludes laser welding, resistance welding, ultrasonic welding or thelike, and is suitably selected depending on the material used.

[0085] (Positive electrode terminal 109)

[0086] Materials for the positive electrode terminal 109 includealuminum and titanium, as is the case with the current collector 104 forthe positive electrode 106. The current collector 104 for the positiveelectrode 106 may be electrically connected to the terminal 109 bylaser-aided welding, resistance welding, ultrasonic welding or the like,depending on the material.

[0087] (Ion conductor 107)

[0088] As the ionic conductor 107 of the lithium secondary battery ofthe present invention, lithium ion conductors such as a separatorholding an electrolyte solution (electrolyte solution prepared bydissolving an electrolyte in a solvent), a solid electrolyte, or asolidified electrolyte obtained by gelling an electrolyte solution witha polymer gel can be used.

[0089] The conductivity of the ionic conductor 107 used in the secondarybattery of the present invention at 25° C. is preferably 1×10⁻³ S/cm ormore, and more preferably 5×10⁻³ S/cm or more. Further, the thickness ofthe ionic conductor 107 is preferably 10 to 40 μm. When the thickness isless than 10 μm, the stress resulting from expansion of the negativeelectrode 103 cannot be absorbed, which is not desirable. On the otherhand, when the thickness is more than 40 μm, the capacity will lower,which is also not desirable.

[0090] As the electrolyte, there may be included, for example, acidssuch as H₂SO₄, HCl or HNO₃; salts of lithium ion (Li⁺) and Lewis acidion (BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻, CF₃SO₃ ⁻ or BPh₄ ⁻ (Ph: phenylgroup)); and a mixture thereof. The salt is preferably treated tosufficiently remove moisture and oxygen by heating under a vacuum or thelike.

[0091] As a solvent for the electrolyte, there may be included, forexample, acetonitrile, benzonitrile, propylene carbonate, ethylenecarbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate,dimethyl formamide, tetrahydrofuran, nitrobenzene, dichloroethane,diethoxyethane, 1,2-dimethoxyethane, chlorobenzene, γ-butyrolactone,dioxolane, sulfolane, nitromethane, dimethyl sulfide, dimethylsulfoxide, methyl formate, 3-methyl-2-oxazolidinone,2-methyltetrahydrofulan, 3-propylsydnone, sulfur dioxide, phosphorylchloride, thionyl chloride, sulfuryl chloride or a liquid mixturethereof.

[0092] Incidentally, it is preferable to either dehydrate theabove-mentioned solvent, for example, with activated alumina, molecularsieve, phosphorus pentoxide or calcium chloride, or depending on thesolvent, to distill the solvent in an inert gas atmosphere in thepresence of an alkaline metal for elimination of impurities anddehydration.

[0093] Further, in order to prevent leakage of the electrolyte solution,it is preferable to use a solid electrolyte or a solidified electrolyte.Here, the solid electrolyte may include a glass material such as anoxide comprising lithium, silicon, oxygen, and phosphorus or sulfurelements, a polymer complex of an organic polymer having an etherstructure. Further, the solidified electrolyte is preferably obtained bygelling the above electrolyte solution with a gelling agent to solidifythe electrolyte solution. It is desirable to use as the gelling agent apolymer that can absorb the solvent of the electrolyte solution toswell, or a porous material capable of absorbing a large amount ofliquid, such as silica gel.

[0094] As the polymer, there may be used polyethylene oxide, polyvinylalcohol, polyacrylonitrile, polymethylmethacrylate,vinylidenefluoride-hexafluoropropylene copolymer, and the like. Further,it is more preferred that the polymers have a cross-linking structure.

[0095] The ionic conductor 107 constituting the separator which playsthe role of preventing short-circuiting between the negative electrode103 and the positive electrode 106 in the secondary battery may alsohave a role of retaining the electrolyte solution and is required tohave a large number of fine pores through which lithium ions can passand to be insoluble and stable in the electrolyte solution.

[0096] Accordingly, as the separator, there are preferably used, forexample, a nonwoven fabric or a micropore structure material of glass, apolyolefin such as polypropylene or polyethylene, a fluororesin, etc.Alternatively, a metal oxide film having micropores or a resin filmcomplexed with a metal oxide may also be used.

[0097] Now, the shape and structure of the secondary battery will beexplained.

[0098] The specific shape of the secondary battery according to thepresent invention may be, for example, a flat shape, a cylindricalshape, a rectangular parallelepiped shape, a sheet shape or the like.Further, the structure of the battery may be, for example, a singlelayer type, a multiple layer type, a spiral-wound type or the like. Ofthose, a spiral-wound type cylindrical battery permits an enlargedelectrode surface area by rolling a separator that is sandwiched betweena negative electrode and a positive electrode, thereby being capable ofsupplying a large current at the time of charging/discharging.Furthermore, batteries having a rectangular parallelepiped shape orsheet shape permit effective utilization of accommodation space inappliances that will be configured by accommodating a plurality ofbatteries therein.

[0099] Now, description will be made in more detail of the shape andstructure of the battery with reference to FIG. 5. FIG. 5 is a sectionalview of a spiral-wound type cylindrical battery. Incidentally, thelithium secondary battery of this shape generally comprise the samestructure as that illustrated in FIG. 1, and has a negative electrode, apositive electrode, an ionic conductor, a battery housing and an outputterminal.

[0100] In FIG. 5, reference numeral 502 denote a negative electrodeactive material layer, 503 a negative electrode, 506 a positiveelectrode, 508 a negative electrode terminal (negative electrode can),509 a positive electrode terminal (positive electrode cap), 507 an ionicconductor, 510 a gasket, 501 a negative electrode current collector, 504a positive electrode current collector, 511 an insulating plate, 512 anegative electrode lead, 513 a positive electrode lead, and 514 a safetyvalve.

[0101] Here, in this spiral-wound type cylindrical secondary battery,the positive electrode 506 having the positive electrode active materiallayer 505 formed on the positive electrode current collector 504 and thenegative electrode 503 having the negative electrode active materiallayer 502 formed on the negative electrode current collector 501 areprovided in opposition to each other via the ionic conductor 507 formedby a separator that retains at least an electrolyte solution therein soas to form a stack of a cylindrical structure rolled up multiple times.Further, the cylindrical stack is accommodated in the negative electrodecan 508 used as the negative electrode terminal.

[0102] Furthermore, the positive electrode cap 509 is disposed as thepositive electrode terminal on a side of an opening of the negativeelectrode can 508 and a gasket 510 is disposed in the remaining parts ofthe negative electrode can 508. Thus, the cylindrical electrode stack isisolated from the positive electrode cap side via the insulating plate511.

[0103] Incidentally, the positive electrode 506 is connected to thepositive electrode cap 509 by way of the positive electrode lead 513.Further, the negative electrode 503 is connected to the negativeelectrode cap 508 by way of the negative electrode lead 512. The safetyvalve 514 is disposed on the side of the positive electrode cap 509 toadjust the internal pressure of the battery.

[0104] Next, an example of assembling procedures for the battery shownin FIG. 5 will be described.

[0105] (1) The ionic conductor 507 as a separator is sandwiched betweenthe negative electrode 503 and the formed positive electrode 506, andassembled into the negative electrode can 508.

[0106] (2) After injection of the electrolyte solution, the positiveelectrode cap 509 is assembled with the gasket 510.

[0107] (3) The assembly obtained in (2) above is caulked.

[0108] The battery is completed in this way. Incidentally, it ispreferable that the above-described preparation of the materials for thelithium battery and assembly of the battery is carried out in dry airfrom which moisture has been removed sufficiently or in a dry inert gas.

[0109] Next, the gasket 510, housing and safety valve 514 constitutingthe above-mentioned lithium secondary battery will be described.

[0110] (Gasket 510)

[0111] As the material of the gasket 510, there may be used, forexample, a fluororesin, a polyolefin resin, a polyamide resin, apolysulfone resin, or a rubber material. Incidentally, the sealing ofthe battery may be conducted by way of glass-sealing, sealing using anadhesive, welding or soldering, besides the caulking using theinsulating packing shown in FIG. 5. As the material of the insulatingplate 511, organic resin materials and ceramics may be used.

[0112] (Battery Housing)

[0113] The battery housing is constituted of the negative electrode can508 and the positive electrode cap 509. As the material of the batteryhousing, stainless steel is preferably used. Further, as other materialsof the battery housing, there are frequently used a titanium cladstainless steel, a copper clad stainless steel or a nickel-plated steel.

[0114] The negative electrode can 508 illustrated in FIG. 5 function asthe battery case and also as a terminal and is therefore preferably madeof stainless steel. However, where the negative electrode can 508 doesnot function as both the battery case and the terminal, in addition tostainless steel, a metal such as zinc, a plastic such as polypropylene,a composite material of a metal or glass fibers and a plastic may beused.

[0115] (Safety Valve 514)

[0116] As the safety valve 514 provided in the lithium secondary batteryin order to ensure safety when the internal pressure in the battery isincreased, for example, rubber, a spring, a metal ball or a rupture diskmay be used.

EXAMPLES

[0117] The present invention is further described in detail by referenceto the following examples.

[0118] A lithium secondary battery of a 18650 size (18 mm in diameterand 65 mm in height) having the sectional structure illustrated in FIG.5 was prepared in each of Examples 1 to 4 of the present invention andComparative Examples 1 to 5 which provide comparative examples for theExamples. The following examples are given for the purpose ofillustration and not by way of limitation.

Example 1

[0119] A lithium secondary battery was prepared in Example 1 by thefollowing procedures.

[0120] 1. Preparation of Negative Electrode 503

[0121]70 parts by weight of an Si—Sn—Cu alloy (weight ratio: 80/15/5)powder with an average particle diameter of .0.5 μm as the activematerial for the negative electrode 503, 15 parts by weight of naturalgraphite with an average particle diameter of 5 μm, 3 parts by weight ofacetylene black, 3 parts by weight of carboxymethylcellulose (in theform of a 2% by weight aqueous solution) and 9 parts by weight ofpolyvinyl alcohol (in the form of a 10% by weight aqueous solution) werekneaded to prepare a paste.

[0122] Next, the paste was applied to both surfaces of a 15 μm thickcopper foil as the negative electrode current collector 501, dried, andpressed and formed by means of a roller press machine to make anelectrode with a capacity per unit volume of the negative electrodeactive material layer 502 of 2,400 mAh/cm³ (capacity per unit weight:2,000 mAh/g; density: 1.2 g/cm³) and a thickness of 16 μm. Thereafter,the thus made electrode was cut into a given size, to which a nickellead was connected by means of spot welding to obtain the negativeelectrode 503.

[0123] 2. Preparation of Positive Electrode 506

[0124] 90 parts by weight of lithium cobaltate, 5 parts by weight ofnatural graphite as a conductive auxiliary material and 5 parts byweight of polyvinylidene fluoride powder were mixed, and then 5% byweight of N-methyl-2-pyrrolidone powder was added thereto. Subsequently,the resulting paste was applied to both surfaces of a 20 μm thickaluminium foil as the positive electrode current collector 504, dried,and then pressed and formed by means of a roller press machine to makean electrode with a capacity per unit volume of the positive electrodeactive material layer 505 of 480 mAh/cm³ (capacity per unit weight: 150mAh/g; density: 3.2 g/cm³) and a thickness of 80 μm. Then, the thus madeelectrode was cut into a given size, to which an aluminum lead wasconnected by an ultrasonic welder, and dried at 150° C. under a vacuumto obtain the positive electrode 506.

[0125] 3. Preparation of Electrolyte Solution

[0126] (1) Ethylene carbonate and diethyl carbonate whose moisture hadbeen sufficiently removed were mixed at a volume ratio of 3:7 to preparea solvent.

[0127] (2) Into the solvent obtained in above (1) was dissolved lithiumtetrafluoroborate (LiBF₄) at a concentration of 1 M (mole/L) to obtainan electrolyte solution.

[0128] 4. Battery Assembly

[0129] The battery assembly was entirely conducted in a dry atmospherecontrolled in moisture with a dew point of −50° C. or less.

[0130] (1) The ionic conductor 507 as the separator was sandwichedbetween the negative electrode 503 and the positive electrode 506, andthe sandwiched member was then spirally wound so as to provide astructure of separator/positive electrode/separator/negativeelectrode/separator, and inserted in the negative electrode can 508 madeof titanium clad stainless steel.

[0131] (2) Next, the negative electrode lead 512 was spot-welded to abottom portion of the negative electrode can 508. Further, aconstriction was formed at an upper portion of the negative electrodecan 508 by means of a necking machine, and the positive electrode lead513 was welded to the positive electrode cap 509 provided with thegasket 510 made of polypropylene by means of a spot welding machine.

[0132] (3) Next, after an electrolyte solution had been injected, thepositive electrode cap 509 was put on, and the positive electrode cap509 and the negative electrode can 509 were caulked with a caulkingmachine to be sealed to thereby prepare the battery.

Example 2

[0133] In this example, a battery was prepared following the sameprocedure as in Example 1 with the exception that the negative electrode503 was prepared which had a capacity per unit volume of the negativeelectrode active material layer 502 of 1,320 mAh/cm³ (capacity per unitweight: 1,100 mAh/g; density: 1.2 g/cm³) and a thickness of 29 μm.

Example 3

[0134] In this example, a battery was prepared following the sameprocedure as in Example 1 with the exception that the negative electrode503 was prepared which had a capacity per unit volume of the negativeelectrode active material layer 502 of 1,920 mAh/cm³ (capacity per unitweight: 1,600 mAh/g; density of 1.2 g/cm³) and a thickness of 12 μm, and85 parts by weight of LiMn₂O₄, 10 parts by weight of carbon black as aconductive auxiliary material and 5 parts by weight of polyvinylidenefluoride powder were used to prepare the positive electrode 506 whichhad a capacity per unit volume of the positive electrode active materiallayer 505 of 270 mAh/cm³ (capacity per unit weight: 100 mAh/g; density:2.7 g/cm³) and a thickness of 80 μm.

Example 4

[0135] In this example, a battery was prepared following the sameprocedure as in Example 1 with the exception that the negative electrode503 was prepared which had a capacity per unit volume of the negativeelectrode active material layer 502 of 2,400 mAh/cm³ (capacity per unitweight: 2,000 mAh/g; density of 1.2 g/cm³) and a thickness of 9 μm, and85 parts by weight of LiMn₂O₄, 10 parts by weight of carbon black as aconductive auxiliary material and 5 parts by weight of polyvinylidenefluoride powder were used to prepare the positive electrode 506 whichhad a capacity per unit volume of the positive electrode active materiallayer 505 of 270 mAh/cm³ (capacity per unit weight: 100 mAh/g; density:2.7 g/cm³) and a thickness of 80 μm.

Comparative Example 1

[0136] In this comparative example, a battery was prepared following thesame procedure as in Example 1 with the exception that the negativeelectrode 503 was prepared which had a capacity per unit volume of thenegative electrode active material layer 502 of 2,700 mAh/cm³ (capacityper unit weight: 2,250 mAh/g; density: 1.2 g/cm³) and a thickness of 14μm.

Comparative Example 2

[0137] In this comparative example, a battery was prepared following thesame procedure as in Example 1 with the exception that the negativeelectrode 503 was prepared which had a capacity per unit volume of thenegative electrode active material layer 502 of 2,720 mAh/cm³ (capacityper unit weight: 1,600 mAh/g; density: 1.7 g/cm³) and a thickness of 14μm.

Comparative Example 3

[0138] In this comparative example, a battery was prepared following thesame procedure as in Example 1 with the exception that the negativeelectrode 503 was prepared which had a capacity per unit volume of thenegative electrode active material layer 502 of 1,040 mAh/cm³ (capacityper unit weight: 1,300 mAh/g; density: 0.8 g/cm³) and a thickness of 37μm.

Comparative Example 4

[0139] In this comparative example, a battery was prepared following thesame procedure as in Example 1 with the exception that the negativeelectrode 503 was prepared which had a capacity per unit volume of thenegative electrode active material layer 502 of 1,080 mAh/cm³ (capacityper unit weight: 900 mAh/g; density: 1.2 g/cm³) and a thickness of 36μm.

Comparative Example 5

[0140] In this comparative example, a battery was prepared following thesame procedure as in Example 1 with the exception that the negativeelectrode 503 was prepared which had a capacity per unit volume of thenegative electrode active material layer 502 of 2,700 mAh/cm³ (capacityper unit weight: 2,250 mAh/g; density of 1.2 g/cm³) and a thickness of 8μm, and 85 parts by weight of LiMn₂O₄, 10 parts by weight of carbonblack as a conductive auxiliary material and 5 parts by weight ofpolyvinylidene fluoride powder were used to prepare the positiveelectrode 506 which had a capacity per unit volume of the positiveelectrode active material layer 505 of 270 mAh/cm³ (capacity per unitweight: 100 mAh/g; density: 2.7 g/cm³) and a thickness of 80 μm.

[0141] (Evaluation of Battery Performance)

[0142] The batteries were evaluated for their performances by thefollowing procedure.

[0143] Each of the prepared batteries was charged at a constant currentof a value of 0.1 C (a current of 0.1 times the capacity/hr) obtained onthe basis of the electrical capacity calculated from the positiveelectrode active material, and when the battery voltage reached 4.2 V,the charging was changed to a constant voltage charging at 4.2 V andcontinued for 10 hours in total. After resting for 10 minutes succeedingthe charging, discharging was performed at a constant current of a valueof 0.1 C (a current of 0.1 times the capacity/hr) until the voltagereached 2.5 V. The battery capacity was defined as the dischargedelectricity amount during the discharging.

[0144] Further, the cycle life was evaluated by performingcharging/discharging cycles with one cycle being constituted ofcharging/discharging at a constant current of 0.5 C (a current of 0.5times the capacity/hr) and a resting period for 20 minutes, andrecording the number of cycles wherein 60% of the battery capacity wasnot reached. Incidentally, the cut-off voltage for charging was set to4.2 V and the cut-off voltage for discharging was set to 2.7 V.

[0145] The results of the battery capacities and charge/discharge cyclelives of the batteries prepared in Examples 1 and 2 and ComparativeExamples 1 to 4 are collectively shown in Table 1, in which the cyclelives for Example 1 and Comparative Examples 1 to 4 are normalized withthe cycle life of the battery of Example 2 is defined as 1.0.

[0146] The results of the battery capacities and charge/discharge cyclelives of the batteries prepared in Examples 3 and 4 and ComparativeExample 5 are collectively shown in Table 2. Further, Table 2 also showsthe defective sample occurrence quantity such as defective coating ofnegative electrode, skewed rolling of electrodes, short circuit and thelike. Incidentally, the cycle life and the defective sample occurrencequantity for each of Example 4 and Comparative Example 5 are normalizedwith the cycle life and the defective sample occurrence quantity of thebattery of Example 3 is defined as 1.0.

[0147] It has been seen from Table 1 that by producing batteries so asto satisfy the conditions defined by the present invention (see FIG. 4),it is possible to stably provide a battery suitable for a specifiedobject of use, such as a battery featuring a high capacity whilesecuring the life to a certain extent as in Example 2, or a batteryfeaturing a long life while still having a higher capacity than acommercially available battery with a graphite negative electrode as inExample 3.

[0148] However, it is noted that the batteries prepared in ComparativeExamples 1 and 2 under such conditions as to go beyond the boundary 2show lowering in cyclic characteristic due to deterioration of negativeelectrode as compared to the battery of Example 1. Further, it is seenthat the batteries prepared in Comparative Examples 3 and 4 under suchconditions as to fall below the boundary 1 each show cycliccharacteristic similar to that of the battery of Example 2 but showconsiderable decrease in battery capacity.

[0149] It is seen from Table 2 that the battery prepared in ComparativeExample 5 under such conditions as to go beyond the boundary 2 showslowering in cyclic characteristic due to deterioration of negativeelectrode as compared to the batteries of Examples 3 and 4. Further, itis seen for the battery prepared in Comparative Example 4 under suchconditions as to go beyond the boundary 3 that because the thickness ofthe negative electrode active material layer 502 is as thin as 10 μm orless, the defective sample occurrence quantity is somewhat greater thanthat of Example 3. TABLE 1 Battery Capacity Normalized (C_(N) ×D_(N))/(C_(P) × D_(P)) (mAh) Cycle Life Example 1 5.0 2,991 0.79 Example2 2.8 2,708 1.0 Comparative 5.3 2,995 0.38 Example 1 Comparative 5.32,987 0.44 Example 2 Comparative 2.1 2,461 1.04 Example 3 Comparative2.3 2,473 1.01 Example 4

[0150] TABLE 2 Defective Sample Battery Occurence capacity NormalizedQuantity as (C_(N) × D_(N))/(C_(P) × D_(P)) (mAh) cycle life NormalizedExample 3 7.1 1,720 1.0 1.0 Example 4 8.9 1,749 0.82 1.3 Comparative10.0 1,761 0.37 1.9 Example 5

[0151] While the present invention has been described with reference toapplication to a cylindrical battery in Examples, it should beunderstood that the present invention is not particularly limited inbattery shape and structure and applicable to lithium ion secondarybatteries of various shapes.

[0152] Further, although a Si—Sn—Cu alloy (weight ratio of 80/15/5)powder has been used in Examples for the negative electrode activematerial, any other alloy particles mainly composed of silicon areexpected to exhibit similar effect.

[0153] As described above, according to the first aspect of the presentinvention, with a lithium secondary battery with a negative electrodecomprising a negative electrode active material layer comprising alloyparticles comprising silicon and tin and having an average particlediameter of 0.05 to 2 μm as an active material, and a current collector,wherein the negative electrode active material layer has a storagecapacity of 1,000 to 2,200 mAh/g and a density of 0.9 to 1.5 g/cm³, itis possible to provide a lithium secondary battery with a high capacityand a long life.

[0154] Further, according to the second aspect of the present invention,with a lithium secondary battery comprising a negative electrodecomprising a negative electrode active material layer comprising alloyparticles as an active material comprising silicon as a major componentand a negative electrode current collector, and a positive electrodecomprising a positive electrode active material layer and a positiveelectrode current collector, wherein the positive electrode activematerial layer and the negative electrode active material layer satisfythe following relationships:

(C _(N) ×D _(N))/(C _(P) ×D _(P))≦8

C _(N) ×D _(N)=1,200 to 2,500 mAh/cm³

C_(N)=1,000 to 2,200 mAh/g

D_(N)=0.9 to 1.5 g/cm³

[0155] (wherein,

[0156] C_(N) represents a capacity per unit weight of the negativeelectrode active material layer;

[0157] D_(N) represents the density of the negative electrode activematerial layer;

[0158] C_(P) represents a capacity per unit weight of the positiveelectrode active material layer; and

[0159] D_(P) represents the density of the positive electrode activematerial layer), it is possible to provide a lithium secondary which canfreely be designed so as to meet either or both of the objects of highcapacity use and long life use.

What is claimed is:
 1. A lithium secondary battery with a negativeelectrode comprising a negative electrode active material layercomprising alloy particles comprising silicon and tin and having anaverage particle diameter of 0.05 to 2 μm as an active material, and acurrent collector, wherein the negative electrode active material layerhas a storage capacity of 1,000 to 2,200 mAh/g and a density of 0.9 to1.5 g/cm³.
 2. The lithium secondary battery according to claim 1,wherein the negative electrode active material layer has a thickness of10 to 50 μm.
 3. The lithium secondary battery according to claim 1,wherein the negative electrode active material layer comprises an activematerial, a binder and a conductive auxiliary material.
 4. The lithiumsecondary battery according to claim 3, wherein at least polyvinylalcohol is used as the binder of the negative electrode active materiallayer.
 5. A lithium secondary battery comprising a negative electrodecomprising a negative electrode active material layer comprising alloyparticles as an active material comprising silicon as a major componentand a negative electrode current collector, and a positive electrodecomprising a positive electrode active material layer and a positiveelectrode current collector, wherein the positive electrode activematerial layer and the negative electrode active material layer satisfythe following relationships: (C _(N) ×D _(N))/(C _(P) ×D _(P))≦8C _(N)×D _(N)=1,200 to 2,500 mAh/cm³C_(N)=1,000 to 2,200 mAh/gD_(N)=0.9 to 1.5g/cm³ wherein, C_(N) represents a capacity per unit weight of thenegative electrode active material layer; D_(N) represents the densityof the negative electrode active material layer; C_(P) represents acapacity per unit weight of the positive electrode active materiallayer; and D_(P) represents the density of the positive electrode activematerial layer.
 6. The lithium secondary battery according to claim 5,wherein the alloy particles comprising silicon as a main component havean average particle diameter of 0.05 to 2 μm.
 7. The lithium secondarybattery according to claim 5, wherein the alloy particles comprisingsilicon as a main component are alloy particles comprising silicon andtin.
 8. The lithium secondary battery according to claim 5, wherein thenegative electrode active material layer has a thickness of 10 to 50 μm.9. The lithium secondary battery according to claim 5, wherein thepositive electrode active material layer has a thickness of 50 to 150μm.
 10. The lithium secondary battery according to claim 5, wherein thenegative electrode current collector has a thickness of 6 to 30 μm. 11.The lithium secondary battery according to claim 5, wherein the positiveelectrode current collector has a thickness of 6 to 30 μm.
 12. Thelithium secondary battery according to claim 5, wherein the negativeelectrode active material layer comprises an active material, a binderand a conductive auxiliary material.
 13. The lithium secondary batteryaccording to claim 12, wherein at least polyvinyl alcohol is used as thebinder of the negative electrode active material layer.